D6394: rust-dirstate: add "dirs" rust-cpython binding

[Yuya Nishihara]

>   To my eyes, the point of `Deref` is to make the `struct` act as a "fat pointer" from the outer interface code, but maybe this does not really help that much?

I feel DirsMultiset isn't a HashMap, but is implemented by using a HashMap.
For example, inner.get().unwrap() returns a refcount of the path, but which
is an implementation detail.

So I think it's better to define methods explicitly instead of proxying
everything to HashMap.